Centrifugal pressure booster and method for modifying or constructing a centrifugal pressure booster

ABSTRACT

The invention provides a centrifugal pressure booster, for pressure boosting liquids, multiphase fluid or gas, the pres sure booster comprising a liquid filled electric motor with a stator and a rotor, with a rotor stator gap between the rotor and stator, a pressure boosting part in the form of a pump or compressor operatively coupled to the rotor, and at least one housing, one fluid inlet and one fluid outlet. The pressure booster is distinctive in that it comprises a rotor stator gap coolant inlet pump, for enhancing the coolant flow through the rotor stator gap. The invention also provides a related method and use of a rotor stator gap coolant inlet pump.

FIELD OF THE INVENTION

The invention relates to centrifugal pumps and compressors, for convenience also termed pressure boosters in this document. More specifically, the invention relates to improved cooling of a pressure booster, which enhances the maximum power and speed and prolong the service life of the pressure booster.

BACKGROUND OF THE INVENTION AND PRIOR ART

Improved cooling of pressure boosters, providing enhanced maximum power and speed and prolonged service life, is of general interest for users of pressure boosters, particularly users of subsea pressure boosters. However, numerous special challenges related to subsea pressure boosting limit the use. Subsea pressure boosting of liquids, multiphase fluid or gas is of great interest for the petroleum industry, due to the huge possibilities made available for the industry. Subsea pressure boosting can increase the production significantly from subsea wells, both the recovery and the production rate, and allow transport of the produced petroleum fluid, processed or unprocessed, to remote onshore or platform locations. Two major challenges for subsea pressure boosting are to improve reliability and to increase the maximum power and speed of the subsea pressure booster.

Motors for pumps or compressors are normally liquid filled. A large friction loss in the gap between the rotor and the stator, especially at high speeds, restricts the rotation of the rotor. The friction in general follows a power of three of the velocity. The friction generates heat, restricting the maximum power rating and rotations per minute and shortening the service life of the pressure booster.

Current practice is to flush a coolant through the gap in order to limit the temperature rise due to the friction, using a common coolant circulation pump serving cooling of the stator windings and the rotor stator gap. More specifically, a single coolant pump flush coolant through the rotor stator gap and also through coolant conduits through or between the stator windings, the flow through the rotor-stator gap and the stator windings conduits being arranged in parallel.

A first known solution to increase the rotor stator gap cooling is to increase the rotor stator gap distance, thereby reducing the resistance for coolant flow. However, this will reduce the efficiency of the magnetic coupling and hence also the motor efficiency. A second known solution to increase rotor stator gap cooling is to add or integrate vanes on the rotor, in the rotor stator gap. Reference is made to the patent publications GB2497667, US2001051097, JPH11230088, JPH1189180 and JP4770441. The teaching of the referred publications is in general unsuitable for the surface speeds related to the current invention.

The objective of the present invention is to provide enhanced maximum power and speed and prolong the service life of a pressure booster. None of the publications mentioned above describes or illustrates an improved or alternative cooling in a centrifugal pressure booster as a means to meet said objective.

SUMMARY OF THE INVENTION

The invention meets the objective by providing a centrifugal pressure booster, for pressure boosting liquids, multiphase fluid or gas, the pressure booster comprising a liquid filled electric motor with a stator and a rotor, with a rotor stator gap between the rotor and stator, a pressure boosting part in the form of a pump or compressor operatively coupled to the rotor, and at least one housing, one fluid inlet and one fluid outlet. The pressure booster is distinctive in that it comprises a rotor stator gap coolant inlet pump, for enhancing the coolant flow through the rotor stator gap.

Preferably, the pressure booster is a subsea pressure booster, further comprising at least one pressure housing and a coolant circulation pump arranged for pumping coolant through said gap and channels through the stator.

In a preferable embodiment, said inlet pump is an impeller fastened to and arranged as an axial extension to laminations or a short circuiting ring of the rotor. In an alternative preferable embodiment, said inlet pump comprises angled blades or vanes fastened to and arranged as an axial extension to laminations or a short circuiting ring of the rotor, or arranged on a rotor shaft adjacent the rotor stator gap.

Preferably, the rotor stator gap coolant inlet pump is a combined balancing ring and impeller, having outer diameter larger than the inner diameter of the rotor stator gap but smaller than the outer diameter of the rotor stator gap, said combined balancing ring and impeller has outlet for coolant directly into the rotor stator gap. However, the outer impeller diameter can be larger than the outer diameter of the rotor stator gap, if a slightly larger diameter external cover or similar directs the flow into the rotor stator gap. Alternatively, the impeller outer diameter can be smaller than the inner diameter of the rotor stator gap, if an external cover or similar directs the flow into the rotor stator gap, which can be a favourable embodiment if cavitation is a possible problem. For state of the art pressure boosters with a common shaft for rotor/motor and pump, the balancing device and impeller is ring shaped. A balancing device, also called balance device, balance ring or balance disc, is used to minimize vibrations and any other possible effects by small misalignments on the shaft where it is attached, by fine tuning weight or extent of material around the rotational axis.

The invention also provides a method for modifying or constructing a centrifugal pressure booster, for pressure boosting liquids, multiphase fluid or gas. For the method, the pressure booster comprises a liquid filled electric motor with a stator and a rotor, with a rotor stator gap between the rotor and stator, a pressure boosting part in the form of a pump or compressor operatively coupled to the rotor, one fluid inlet and one fluid outlet, and at least one pressure housing if the pressure booster is for subsea operation. The method is distinctive by providing the pressure booster with a rotor stator gap coolant inlet pump, for enhancing the coolant flow through the rotor stator gap.

Preferably, a combined balancing ring and impeller is arranged as the rotor stator gap inlet pump, preferably having outer diameter larger than the inner diameter of the rotor stator gap but smaller than the outer diameter of the rotor stator gap, said combined balancing ring and impeller being arranged having outlet directly into the rotor stator gap and preferably it is fastened to and arranged as an extension to laminations or a short-circuiting ring of the rotor, as a ring on a rotor shaft.

The invention also provides use of a rotor stator gap coolant inlet pump in a pressure booster, preferably a subsea pressure booster, for enhancing the coolant flow through a rotor stator gap of the pressure booster.

The invention provides balancing of the flow rate through the stator windings and the rotor-stator gap, which will have very different frictional characteristics and hence different pressure drops. The invention ensures that at all relevant rotational speeds, both the stator windings and the rotor-stator gap have sufficient liquid coolant flow, providing enhanced maximum power and speed and prolonged service life of the pressure booster of the invention. Said inlet pump rotates with the rotor, without disturbing the rotor stator gap flow by increasing the friction, thereby solving what is assumed to be a major problem with prior art devices with vanes in all of or at least in a significant length along the rotor stator gap.

The term a “rotor stator gap coolant inlet pump”, in this context means vanes or blades or similar structural elements arranged at the motor stator gap inlet, as well as impellers with at least one blade, arranged not into the motor stator gap as seen in radial direction, but at the inlet thereof, just outside the gap. This means that the coolant flow exits directly from the outlet of said inlet pump into said gap inlet and said inlet pump is arranged adjacent to said gap, which is just besides the radial motor stator gap without any significant axial distance between, for enhancing the coolant flow through the rotor stator gap. Axial means parallel to the rotor rotation axis, radial means radial to the rotor rotation axis.

With the term an “impeller”, it is in this context meant a device typically having a radial fluid displacement component upon rotation, as provided by having at least one blade or fluid conduit. The fluid inlet of an impeller typically is nearer the rotation axis than the fluid outlet. With the terms a “blade” or “vane” it is meant an axial fluid displacement component shaped as a blade or similar, as seen in the prior art publications, but for the present invention not arranged in the rotor stator gap. The rotor stator gap coolant inlet pump may however comprise elements of any operative kind providing pumping effect when rotating.

As skilled persons may realize, the coolant of the motor of the pressure boosters of the invention is a liquid, the rotor stator gap has in substance smooth, even surfaces, without rotor blades as seen in prior art solutions, and the pressure booster typically operates at high speed and power, such as 2000-6000 rpm (rotations per minute) and power counted in megawatts.

The blades or vanes are angled or skew in order to provide pumping effect upon rotation. Preferably, the blades are optimized with respect to shape and number for sufficient pumping effect at the intended operating conditions, such as a rotation speed of 6000 rpm. The at least one blade is made with an angle to the tangential direction, so upon rotating the pump or impeller device, as attached to the rotor laminations or rotor shaft or both, a predictable coolant flow component parallel to the rotation axis is generated, enhancing coolant flow through the rotor stator gap.

Without wishing to be bound by theory, it is assumed that the prior art solution of arranging vanes in the rotor stator gap dramatically increases the friction. Accordingly, the flow resistance and heat generation in the rotor stator gap become very high with the state of the art solutions.

The solution of the present invention is also much simpler than the prior art solution with respect to machining and installation. For the most preferred embodiment, a combined balancing ring and impeller will preferably be made by a special wear resistant steel or brass or alloy or other material more resistant to wear and preferably also more feasible for machining and fabrication, than the rotor shaft and laminations.

The invention assures a steady flow of coolant through the rotor stator gap, which better will remove the frictional heat in the gap. This provides a prolonged lifetime of the motor, enhanced power rating and maximum rpm for the pressure booster, and simplifies the fabrication, installation and maintenance of the pressure booster compared to having blades in the whole or a significant length of the rotor stator gap.

FIGURES

FIG. 1 illustrates a subsea pressure booster of the invention, with a combined balancing device and rotor stator gap circulation impeller.

FIG. 2 is an illustration of a detail of a subsea pressure booster of the invention.

DETAILED DESCRIPTION

Reference is made to FIG. 1, illustrating, in longitudinal section, a subsea pressure booster 1 of the invention, with a combined balancing device and rotor stator gap circulation impeller 2. FIG. 2 illustrates the motor part of the subsea pressure booster of FIG. 1 in more detail. Accordingly, in the illustrated embodiment, the rotor stator gap coolant inlet pump is a combined impeller and balancing device. In some otherwise identical or similar embodiments, the rotor stator gap coolant inlet pump is not a combined impeller and balancing device. FIG. 2 illustrates the impeller 2 in detail, and it can be seen clearly that the impeller comprises a number of blades 2 b. The impeller is fastened to the rotor 3 at the inlet of the rotor stator gap, as an axial extension of the rotor laminations/short circuit ring. The impeller has outer diameter just smaller than the outer diameter of the rotor stator gap, to ensure clearance at different temperatures. The outlet outer diameter of the impeller hub, not the blades, is identical to the outer diameter of the short circuiting ring and rotor laminations. Around the rotor 3 is a stator 5, between the rotor and stator is the rotor stator gap 6, which is an annular volume with smooth radial inside and outside surfaces, without vanes increasing the friction for flow. Furthermore, the figure illustrates a coolant circulation pump 7 arranged for pumping coolant through said gap and stator channels, in the form of a common circulation impeller 7 feeding both stator channel and rotor stator gap coolant flow. A common, prior art type circulation impeller 7 is illustrated, and the rotor stator gap cooling flow 8 and the stator channels cooling flow 9.

The centrifugal subsea pressure booster of the invention can include any feature or step as here illustrated or described, in any operative combination, each such combination is an embodiment of the invention. The method of the invention can include any feature or step as here illustrated or described, in any operative combination, each such combination is an embodiment of the invention. The use of the invention can include any feature or step as here illustrated or described, in any operative combination, each such combination is an embodiment of the invention. 

1. A centrifugal pressure booster, for pressure boosting liquids, multiphase fluid or gas, the centrifugal pressure booster comprising: a liquid filled electric motor with a stator and a rotor, with a rotor stator gap between the rotor and stator; a pressure boosting part in the form of a pump or compressor operatively coupled to the rotor; and at least one housing, one fluid inlet and one fluid outlet; and wherein the centrifugal pressure booster comprises a rotor stator gap coolant inlet pump, for enhancing the coolant flow through the rotor stator gap.
 2. The centrifugal pressure booster according to claim 1, wherein the centrifugal pressure booster is a subsea centrifugal pressure booster for pressure boosting liquids, multiphase fluid or gas at subsea locations, the centrifugal pressure booster further comprising at least one pressure housing and a coolant circulation pump arranged for pumping coolant through the gap and stator channels,
 3. The centrifugal pressure booster according to claim 1, wherein the inlet pump is an impeller fastened to and arranged as an axial extension to laminations or a short circuiting ring of the rotor.
 4. The centrifugal pressure booster according to claim 1, wherein the inlet pump comprises angled blades or vanes fastened to and arranged as an axial extension to laminations or a short circuiting ring of the rotor, or arranged on a rotor shaft adjacent the rotor stator gap.
 5. The centrifugal pressure booster according to claim 1, wherein the rotor stator gap coolant inlet pump is a combined balancing ring and impeller, having outer diameter larger than the inner diameter of the rotor stator gap but smaller than the outer diameter of the rotor stator gap, the combined balancing ring and impeller has outlet for coolant directly into the rotor stator gap.
 6. A method for modifying or constructing a centrifugal pressure booster, for pressure boosting liquids, multiphase fluid or gas, the centrifugal pressure booster comprising a liquid filled electric motor with a stator and a rotor, with a rotor stator gap between the rotor and stator, a pressure boosting part in the form of a pump or compressor operatively coupled to the rotor, one fluid inlet and one fluid outlet, and at least one pressure housing if the centrifugal pressure booster is for subsea operation, the method comprising: providing the centrifugal pressure booster with a rotor stator gap coolant inlet pump, for enhancing the coolant flow through the rotor stator gap.
 7. The method according to claim 6, wherein a combined balancing ring and impeller is arranged as the rotor stator gap inlet pump, having outer diameter larger than the inner diameter of the rotor stator gap but smaller than the outer diameter of the rotor stator gap, the combined balancing ring and impeller being arranged having outlet directly into the rotor stator gap and preferably it is fastened to and arranged as an extension to laminations or a short-circuiting ring of the rotor, as a ring on a rotor shaft.
 8. (canceled) 